1.Selection of electrodes
Reference factors: electrode type, electrode spacing.
(1) Selection of the type of excitation electrode There are many types of excitation electrodes used for emission spectrum analysis, such as carbon, copper, aluminum, tungsten, silver, etc. Generally, different excitation electrodes are selected according to different analysis methods and analysis objects. The principle is that the selected electrode type should have better analysis accuracy in the analysis results; the elements to be analyzed should not be in the exciting electrode material; the electrode corrosion should be small; in daily analysis, it should be used repeatedly. Increase analysis speed. For example, steel analysis usually does not include steel or analytical silver. Silver is used as the excitation electrode. The accuracy of the analysis results is relatively high. The silver electrode tip should be tapered and the tip should be 90 °. For example, for an excitation light source with a unidirectional discharge, the excitation electrode is easily corroded during the discharge. Therefore, tungsten rods are used as excitation electrodes, and tungsten electrodes are generally not prone to long tips and do not require continuous cleaning of the electrode hundreds of times.
(2) Selection of electrode spacing The size of electrode spacing has a great influence on the accuracy of analysis. The electrode spacing is too large, the stability is poor, it is difficult to excite, and the accuracy is poor. The electrode spacing is too small, although it is easy to excite, as the number of discharges increases, the condensed state of the auxiliary electrode increases, which often causes long sides and affects the accuracy of the analysis, especially the accuracy of components that are sensitive to changes in spacing is poor. Therefore, the electrode spacing should not be too large or too small, and the typical analysis distance is 4 to 5 mm. The electrode spacing is usually not selectable.
2.Selection of flushing, pre-burning and exposure time
(1) The purpose of flushing and pre-burning flushing is to minimize the air in the sample excitation table, especially O2, H2O, etc., which have an adverse effect on the excitation. General analysis of non-ferrous metals such as aluminum can be used for 2s, analysis of ferrous metals can be used for 3s. Rinse time should not be too long, so as not to consume too much argon and prolong the analysis time.
(2) Exposure time Exposure time mainly depends on the reproducibility of elemental analysis in the excited sample. The exposure process is the process of charging (also called integration) the photocurrent into the integrating capacitor. The result of integration can be considered as the average value of the photocurrent, so the integration time should not be too short. In order to ensure the analysis accuracy, the total number of spark discharges should be between 2000 and 3000 times, so that the light intensity value and ratio of iron to the analysis element are relatively moderate. In normal analysis, the exposure time is generally 3 ~ 5s. But it must be pointed out that the length of exposure time is related to the energy of the light source.
3.Selection of high and low standard samples
There are many factors that cause curve drift in daily analysis: the lens is contaminated to form a coating, the long tip of the electrode during excitation will increase the curve display background, and argon flow, pressure, purity, and changes in room temperature environment will cause the curve to drift Therefore, it is important to correct the working curve frequently. It is not easy to select a set of standardized samples, including the high content and low content of all the elements to be analyzed. These standardized samples must be uniform and the data of the excitation spectrum analysis must be highly reproducible. At the same time, the number of standardized samples should be as small as possible, because standardized samples are expensive, and each sample requires additional time.
In single-point standardization, only high or low content standards are required. If the sample covers a wide range of content and has high accuracy at low levels, then two points of standardization are required. At this time, there are two standard samples, one for high standards and one for low standards. Or each standard contains some high-concentration elements and other low-concentration elements, as long as they can include all elements.
